The production of hybrid circuits on substrates, such as aluminum oxide and aluminum nitride, is conventionally performed using sputtering techniques. Thin films of materials can be uniformly deposited on substrates by evaporation or sputter deposition. Sputter deposition is a far more complex process than evaporation, requiring more care and skill for good process control. The adhesion of the thin film to the substrate is significantly better with sputtering than with vacuum evaporation. This is due both to enhanced substrate cleanliness and to the very energetic sputtered atoms. In sputtering, the material to be coated as a thin film is dislodged from the source or target by the impact of controlled, low pressure inert gas ions accelerated by a potential of five hundred to five thousand volts. The dislodged atoms deposit on a substrate which is located at the anode or in a separate holder. This mechanism of vapor generation in sputtering through high energy impact results in its unique ability to deposit films of materials having very low vapor pressure (high melting point). Since sputtering rates are, in general, similar for different metals and alloys, sputtering provides better compositional control of complex coating materials than vacuum evaporation. The substrate is typically cleaned by exposing it to a plasma-glow discharge sufficient to remove organic contaminants from the surface. The substrate is then coated with metal by sputtering or evaporating a thin layer of chromium or titanium as a bonding layer, and further vacuum metallizing a layer of copper over the bonding layer before any significant oxidation of the chromium or titanium can occur. In some instances, a layer of nickel is vacuum deposited between the chromium and the copper layers. In this case, both the nickel and the copper layer are deposited before any significant oxidation of the chromium or titanium layer can occur. The steps of this process are preferably performed one after the other in the same evacuated reactor without breaking the vacuum or admitting any oxygen into the reaction chamber. When this operation is complete, the substrate is removed from the chamber and is ready for subsequent electroplating, circuit definition, and other treatments, as may be desired to develop the appropriate circuitry. A typical procedure for treating substrates is outlined in U.S. Pat. No. 4,604,168 issued to Liu and Lindsay on Aug. 5, 1986, and addresses the need to exclude oxygen from the chamber during sputtering.
Unfortunately, this process of sputtering an adherent layer of chromium to the substrate requires that the chromium interact with the surface of the substrate in order to form a bond to the substrate. When vacuum metallizing substrates such as aluminum nitride, adhesive failure is frequently seen, due to a separation between the chromium and the aluminum nitride surface. This is thought to be due to a lack of interaction with the aluminum nitride. Therefore, we find that aluminum nitride does not provide proper adhesion in the conventional thin film sputtering processes.
Aluminum nitride substrates show poor adhesion of the metal as exhibited by peeling of the metal away from the substrate and blistering of the metal. It would be highly advantageous to have a method of metallizing thin films to aluminum nitride that would provide improved adhesion and eliminate blistering for the formation of hybrid electronic circuits.